Scala - type mismatch; found Int, required String

Question

I have a problem with type mismatch in Scala. Compiler returns an error:

error: type mismatch; found : Int required: String val v3:Int = (v1+v2)

This is code of my function:

    def sumMaps[T, Int](m1: Map[T, Int], m2: Map[T, Int]): Map[T, Int] = {
        val sameKeys = m1.keySet.intersect(m2.keySet)
        var m3: Map[T, Int] = m1 ++ m2
        for (key <- sameKeys) {
            val v1:Int = m1(key)
            val v2:Int = m2(key)
            val v3:Int = (v1+v2)
            //val v:Int = (m1(key) + m2(key))
            m3 = m3 updated (key, v3)
        }
    m3
}

Why? Any idea what could be the problem?

David Frank David Frank · Accepted Answer · 2015-02-04T10:02:51

You are redeclaring Int as a type parameter when it is a concrete type.

You should change the method head to:

def sumMaps[T](m1: Map[T, Int], m2: Map[T, Int]): Map[T, Int] = {

When Int is a type parameter, the compiler replaces (v1+v2) with a string addition, so it becomes (v1.toString + v2.toString), this gave you the problem.

UPDATE

I came up with a simpler solution to your problem:

def sumMaps[T](m1: Map[T, Int], m2: Map[T, Int]): Map[T, Int] = {
  (for (key <- m1.keySet ++ m2.keySet) yield {
    val v1: Int = m1.getOrElse(key, 0)
    val v2: Int = m2.getOrElse(key, 0)
    key -> (v1 + v2)
  }).toMap
}

Or an even simpler one:

def sumMaps[T](m1: Map[T, Int], m2: Map[T, Int]): Map[T, Int] = {
  (m1 ++ m2).transform {
    case (key, _) => m1.getOrElse(key, 0) + m2.getOrElse(key, 0)
  }
}

Scala - type mismatch; found Int, required String

1 Answers